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Week 2 (belated)

Part 2.5 – Responses to questions

What are climate change records?
Records range from modern instruments on satellites and balloons and ships, through older observational records dating back perhaps a few hundred years of temperature and weather on land and sea, to indirect records of climate preserved in tree rings and ice cores. All data sets need comparing with others to remove biases in measurement. For example, rings from numerous trees are required as a single tree might have had a particular life history (e.g. parasite infestations) which also affect its rings. Satellite measurements of sea surface temperature (measured indirectly through radiation) can be affected by conditions in the atmosphere and might differ from a direct measurement using a thermometer in the water.

How do volcanoes affect climate change?
Volcanoes are a major source of aerosols in the atmosphere. Aerosols are tiny particles which in their effect on climate can be divided into two types: bright particles which tend to reflect the sun’s radiation and thus cause cooling, and dark particles which tend to absorb radiation and thus cause warming. The current view is that the cooling effect is larger than the warming effect from aerosols, overall. However, aerosol concentrations and types vary across the globe and their effect is localised, so this is a complex area which is not yet understood. Volcanoes also eject CO2, a greenhouse gas, which long-term has a warming effect (see snowball earth discussions).

How is today’s warming different from the past’s?
Primarily, there is much more rapid warming, about ten times faster over the last century than over the previous 5000 years moving out of the Ice Age. Warming is predicted to be even faster (20 times) over the next century.

What is the role of isotopes in determining temperatures from the past?
Calcium carbonate, CaCO3, is formed by small sea creatures which die and fall to the sea bed, forming layers of calcium carbonate rock (eg White Cliffs of Dover). The oxygen atoms occur naturally as two isotopes – 16O and the heavier, less common 18O. The ratio of the isotopes used to form the shells of the sea creatures varies very slightly with temperature – 0.2 parts per million decrease for 1C increase in temperature. In principle, therefore, the variation in oxygen isotopes in calcium carbonate rocks can be used to determine the temperature at the time the rocks were laid down. In practice, the measurement is complicated by other factors. The ratio in the shells is critically dependent on the ratio in the surrounding seawater, and this varies as much as with temperature. Models have been built which are beginning to unravel the effects of the different factors.

How have trees been used to reconstruct different climate variables across the world?
Trees produce a ring of growth each year during the growing season. The amount and type of growth varies according to the age of the tree, to conditions such as rainfall and temperature, and also to ‘stand dynamics’ – the competition for light and nutrients with other trees and plants. The occurrence of disturbance factors such as nearby trees being blown down or an infestation of insects will also affect the rate of growth. Tree rings will have different widths and densities of wood depending on how limited they were by the various factors. Low rainfall, for example, will lead to reduced growth and narrow rings. In a set of tree rings from a single tree, the precise cause of the nature of the ring for any year will be an accumulation of factors which cannot be differentiated. However, by examining a number of trees in various areas, variations due to disturbance factors can be eliminated and the effects of temperature and rainfall can be isolated.

How can ice cores provide a record of atmospheric composition?
Ice cores give information on atmospheric composition in three ways.
(i) Snow forms around particulates in the atmosphere, such as dust, volcanic ash, smoke and pollen, trapping them and bringing them to the ground as it falls.
(ii) When snow settles on ice, air is in the space between flakes. When more snow falls on top, the air is compressed and tiny bubbles form, preserving a record of the gases in the air.
(iii) Wind-blown dust and volcanic ash can settle on the surface of the ice and get fixed in that layer by subsequent snow falls and ice formation. Analysis of the dust can show where it came from and thus show the wind patterns that brought it there.

2 thoughts on “Week 2 (belated)”

My name is Laura and I am also doing the course on climate change and came across your website by searching question 5 on the internet. It brought me straight here! I only started last week, so I am a bit behind everyone else and I am trying to catch up. I have come to this course completely ignorant. I know nothing at all about climate change but I saw this course and thought I ought to educate myself about what is happening to our planet. I must admit I am already finding it difficult! I will persevere though. I have a blog which I have just started and I am attempting to answer these questions there. The one on isotopes was too much for my little brain to handle! I am currently researching the question on trees. Your answers are very clear and succinct, so maybe you will help me understand some of these issues more clearly. Anyway, just wanted to say hello and thanks!